1. Field of the Invention
The invention relates to a method of manufacturing cis-2,6-dimethylmorpholine by isomerizing trans-2,6-dimethylmorpholine.
2. Discussion of Background
In the synthesis of dimethylmorpholine by the dehydration of diisopropanolamine in the presence of acid catalysts such as sulfuric acid (U.S. Pat. No. 3,083,202; Eur. Pat. 0 094 565; "Houben-Webyl", Vol. 6/4, pp. 510-520), mixtures of cis and trans isomers are always produced. Often it is the cis isomer which is preferred, for example, because of the higher effectiveness of plant protection agents or vulcanization accelerators produced from it (Ger. OS 26 57 747, OS 27 52 096, OS 27 52 135; U.S. Pat. No. 3,083,202). Accordingly, there is a demand for means of converting trans-2,6-dimethylmorpholine (formula I): ##STR1## into cis-2,6-dimethylmorpholine (formula II): ##STR2##
A number of means are known for increasing the content of the cis isomer in a cis/trans-2,6-dimethylmorpholine mixture. One such method is the isomerization of the mixture in an excess of concentrated or fuming sulfuric acid at 180.degree.-220.degree. C. (U.S. Pat. No. 3,083,202). Also, by appropriate control of the process, the cis-content can be increased at temperatures of only 150.degree.-190.degree. C.; however, this requires a substantial molar excess of sulfuric acid (Eur. Pat. 0 094 565).
Disadvantages of both of these methods include the necessary presence of alkali sulfates which are produced in the neutralization of the morpholine base, and yields which decrease (accompanied by increased by-product formation) with increasing degree of isomerization.
There are also various catalytic isomerization methods employing metallic hydrogenation catalysts in the presence of hydrogen which do not produce appreciable amounts of by-products and waste products. Thus, when one or more metals from group VIII or IB of the periodic table are employed, with temperatures of, in particular, 150.degree.-250.degree. C., in the most favorable cases isomerizations are attained which yield an isomer ratio (as given in Eur. Pat. 0 094 565) of 88:12 (cis/trans) after establishment of thermodynamic equilibrium.
According to the method of Eur. Pat. 0 007 520, Examples 7-8, conversions of 76.7 and 86.2%, with selectivities of 92.3 and 90.9%, respectively, were achieved after distillation, using a Pd/Pr.sub.2 O.sub.3 catalyst, at temperatures of 230.degree. and 250.degree. C., respectively. Throughputs of 60 parts trans-2,6-dimethylmorpholine per hr and 10,000 vol parts hydrogen per hr were employed in a continuous flow system with 500 vol parts catalyst. Palladium showed superior effectiveness among the metals claimed.
Also, in Eur. Pat. 0 026 367, catalytic isomerization on metal catalysts comprised of Pd, Zn, Cd, and Mn is described, and in Eur. OS 0 129 904 the use of Pt, Ru, or Rh is described, wherein according to Example 1 of the former patent, with a catalyst comprising Pd, Zn and Cd, a conversion of 74.5% was achieved, with a selectivity of 94.6%, after distillation. According to Example 1a of the former patent, with a Pt catalyst, conversion 81.9% and selectivity 97.2% were achieved after distillation.
Here the throughput was 100 parts trans-2,6-dimethylmorpholine per hr, with 300,000 vol parts hydrogen per hr, over 1000 vol parts catalyst (0.1 parts trans-2,6-dimethylmorpholine with 100 vol parts hydrogen, over 1 vol part catalyst). The conversion and selectivity figures were calculated from the literature data, using the definitions in "Ullmanns Encyklopaedie der technischen Chemie", 4th Edition, Vol. 13, p. 550.
The isomerization catalysts according to Eur. Pats. 0 007 520 and 0 026 367 have the disadvantage of a rapid drop in activity because of the presence of sulfur compounds in technical 2,6-dimethylmorpholine. These sulfur compounds are inevitable impurities from the manufacturing process. The only exceptions are the last-mentioned noble metal catalysts based on Pt, Ru, or Rh. A further disadvantage of all three above-mentioned methods is the high price of the noble metal catalysts.